Combustion Chamber for Burning Solid Fuels

ABSTRACT

A combustion chamber ( 10 ) for burning solid fuels having a high volatiles content has a hollow body ( 11 ) with a cylindrical wall section ( 12 ) and a top section ( 13 ) located at end ( 14 ) of the cylindrical wall section ( 12 ). The hollow body ( 11 ) has a primary air inlet ( 20 ), a fuel inlet ( 23 ) and four volatiles outlets ( 25 ) (one shown), mounted therein. Each volatiles outlet ( 25 ) has a plurality of apertures ( 26 ) of differing sizes arranged in a plate ( 27 ). In use, the burning volatiles exit the apertures ( 26 ) in a turbulent flow resulting in efficient combustion of the volatiles. Additionally, a heat transferring surface ( 33 ) on a water jacket ( 32 ) is shaped around the combustion chamber ( 10 ) so as to define a channel ( 34 ) there between. This arrangement results in an efficient transfer of heat from the turbulent flow of volatiles, which is forced to circulate around the channel ( 34 ).

TECHNICAL FIELD

This invention relates to a combustion chamber for burning solid fuelshaving a volatiles content and, in particular, to a combustion chamberfor burning solid fuels having a high volatiles content, and also to afire chamber incorporating such a combustion chamber.

By solid fuels having a high volatiles content in this context is meantsolid fuels such as wood pellets, pellets made from switchgrass,miscanthus, maize stalks, straw or the like, and nut shells such asalmond shells, all collectively referred to as solid biomass fuels, andalso fossil fuels such as peat or bituminous coal. In contrast, anexample of a low volatiles solid fuel would be anthracite coal.

BACKGROUND ART

In recent years, worldwide concern has arisen relating to climaticchanges ascribed to the increase in atmospheric carbon dioxide releasedby fossil fuels. The price of such fuels has been increasing, andprojections as to the remaining world supplies of such fossil fuels haveled to increased interest in the development of devices utilisingalternative fuels. Furthermore, use of renewable fuels in such devicescould slow down the increase in carbon dioxide levels in the atmosphere.

Devices, which burn wood are known. However, cord wood burnsinefficiently in conventional combustion chambers and is inconvenient touse in comparison with oil or gas burning devices.

The use of biomass fuels is also increasing. However, again, biomassfuel tends to burn inefficiently in conventional devices and this is dueto its high volatiles content. Much of the heat is released into theburning gases (the flames) and is lost up the chimney or flue.

Another reason that conventional boilers and stoves are unsuitable forburning pelleted biomass fuels such as wood pellets, wood chips, etc.,is that such pellets tend to smoulder at reduced output particularlywhen they have a relatively high moisture content. Consequently deviceshave been developed for burning these fuels more efficiently.

Thus, for example, a typical wood pellet stove includes a hopper, anauger, a firebox or grate, a combustion fan and a heat exchanger which,respectively, store, feed, burn the fuel and transfer the heat to thespace to be heated. The auger operates in a timed manner to control thedelivery of the pellet fuel from the hopper into the firebox. The rateof delivery of the fuel to the firebox is matched to the rate ofconsumption of the fuel for a particular output. The combustion fanprovides a measured amount of air to the firebox. An example of such astove is the Pellet stove Mod. 1000 manufactured by CaminettiMontegrappa s.r.l of via A. da Bassano, July 9, 36020 Pove Del Grappa(VI), Italy.

However, a problem with such stoves and boilers is that the transfer ofheat from the burning volatiles is relatively low, particularly at lowoutput, and acceptably high efficiencies can only be achieved by passingthe flue gases through extensive heat exchangers.

It is an object of the present invention to overcome the disadvantagesof the devices hereinbefore described.

DISCLOSURE OF INVENTION

Accordingly, the invention provides a combustion chamber for burningsolid fuels having a high volatiles content, the combustion chambercomprising an enclosed hollow body in which the fuel is to be burnt, thebody having a fuel inlet, a primary air inlet, a secondary air inlet anda volatiles outlet mounted therein, the volatiles outlet having aplurality of apertures, such that, in use, the burning volatiles exitthe apertures in a turbulent flow resulting in efficient combustion ofthe volatiles.

An advantage of the combustion chamber according to the invention isthat the volatiles have to exit the chamber via the apertures in thevolatiles outlet and this causes an increase in the velocity of theburning volatiles as they exit. It also causes the turbulent flow ofvolatiles. The result is that the volatiles burn more efficiently and ata higher temperature than is achieved in a conventional device burningthe same fuel. A consequence of this greater efficiency is that thelevels of harmful products, such as the nitrous oxides, in the fluegases are reduced relative to known devices.

A combustion chamber according to the invention can be used in devicessuch as boilers, air heaters and stoves, in hotplates, in devices forproviding a source of heat for an industrial process, in an incineratoror the like.

Preferably, the apertures are of differing sizes.

Having apertures of differing sizes in the volatiles outlet results inmore efficient mixing of the volatiles in the turbulent flow leading tomore efficient combustion.

The apertures can be arranged in a particular pattern so as to optimisethe performance of the combustion chamber in a particular application.

Further, preferably, the secondary air inlet is adjacent the volatilesoutlet.

The positioning of the secondary air inlet can be important for aparticular application of the combustion chamber and such positioningwill affect the characteristics of the turbulent flow exiting from thevolatiles outlet.

In one embodiment of the invention the volatiles outlet has means fortemporarily restricting the flow of volatiles therethrough.

An advantage of the restricting means is that efficient combustion canbe maintained at varying outputs. Thus, at low output the volatilesoutlet can be restricted so as to maintain a turbulent flow of volatilestherethrough.

Preferably, upstanding formations on the outer surface of the bodyadjacent the apertures are shaped so as to direct the emerging volatilesalong the outer surface.

By diverting the burning volatiles along the outer surface of the bodythe body is maintained at a higher temperature which leads to moreefficient combustion of the fuel. This is a requirement at low outputparticularly when the fuel has a relatively high moisture content.

In circumstances where the problem of high moisture content in the fuelis acute, the primary air can also be preheated by the burning volatilesby passing the primary air supply duct through or against the burningvolatiles.

In a further embodiment of the invention, two or more volatiles outletsare mounted in the hollow body.

The provision of a number of volatiles outlets results in more evendistribution of the heat from the burning fuel.

This arrangement also lends itself to maximising the efficiency of thecombustion chamber at all available outputs, while restricting theemission of oxides of nitrogen by reducing the peak temperature ofcombustion.

In another aspect of the invention there is provided a fire chamber fora device for burning solid fuels having a high volatiles content, thefire chamber comprising a housing, a combustion chamber within thehousing, the combustion chamber having an enclosed hollow body in whichthe fuel is to be burnt, the body having a fuel inlet, a primary airinlet, a secondary air inlet and a volatiles outlet mounted therein, thevolatiles outlet having a plurality of apertures, such that, in use, theburning volatiles exit the apertures in a turbulent flow resulting inefficient combustion of the volatiles.

By employing the combustion chamber with the volatiles outlet, whichcauses a turbulent flow in the emerging volatiles, the fire chamber isheated more efficiently than a conventional fire chamber consuming asimilar amount of fuel.

Preferably, an inner heat transferring surface of the housing is shapedabout the combustion chamber so as to define a channel therebetween,such that, in use, the burning volatiles exiting the apertures areforced to circulate around the combustion chamber within the channel.

An advantage of this arrangement is that the heat released by thevolatiles is transferred both to the heat transferring surface and backinto the combustion chamber. This provides a means for increasing thetemperature of the zone into which the fresh fuel is introduced,particularly at low output, resulting in an increase in the overalltemperature of combustion.

In one embodiment of the fire chamber according to the invention, a setof upstanding curved formations is mounted on the inner heattransferring surface around the combustion chamber within the channel.

The shape and positioning of the set of upstanding curved formationswithin the channel further directs the circulation of the volatilesaround the combustion chamber and also enhances the turbulent flow ofthe volatiles.

In a further embodiment of the fire chamber according to the invention,the apertures are of differing sizes.

In a further embodiment of the fire chamber according to the invention,the secondary air inlet is adjacent the volatiles outlet.

In a further embodiment of the fire chamber according to the inventionthe volatiles outlet has means for temporarily restricting the flow ofvolatiles therethrough.

Preferably, upstanding formations on the outer surface of the bodyadjacent the apertures are shaped so as to direct the emerging volatilesalong the outer surface.

In a further embodiment of the fire chamber according to the invention,two or more volatiles outlets are mounted in the hollow body.

This arrangement results in the efficient circulation of the burningvolatiles around the combustion chamber.

At certain outputs, the tail of the flame emerging from each volatilesoutlet will run into the flame emerging from the next volatiles outlet.This arrangement can provide a means for achieving complete combustionat the tail of each flame. It can also cause a reduction in the peaktemperature of combustion, thereby preventing the formation and emissionof nitrous oxides.

Preferably, the volatiles outlets are arranged equidistantly around thesurface of the hollow body.

The equidistant arrangement of the volatiles outlets optimises the flamemerging effect described above.

Alternatively, the volatiles outlets are positioned about the surface ofthe hollow body so as to facilitate the optimisation of the turbulentflow of volatiles thereabout for each output setting.

Thus, for a particular output setting the volatiles flow through aselected number of the volatiles outlets could be restricted so as tooptimise the flame merging effect while minimising the emission ofnitrous oxides.

In a further embodiment of the fire chamber in accordance with theinvention, means for moving the combustion chamber towards and away fromthe inner heat transferring surface is provided.

An advantage of this arrangement is that when a low output is requiredthe combustion chamber can be moved closer to the inner heattransferring surface. This has the effect of restricting the flow ofvolatiles through the volatiles outlet. Conversely, when a higher outputis required the combustion chamber can be moved further away from theinner heat transferring surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further illustrated by the following descriptionof embodiments thereof, given by way of example only with reference tothe accompanying drawings in which:

FIG. 1 is a vertical section through a boiler containing a combustionchamber and a fire chamber in accordance with the invention;

FIG. 2 is a horizontal section on line II-II of FIG. 1;

FIG. 3 is a vertical section through a second embodiment of a combustionchamber in accordance with the invention;

FIG. 4 is a vertical section through a third embodiment of a combustionchamber in accordance with the invention; and

FIG. 5 is a horizontal section on line V-V of FIG. 4.

MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 there is illustrated, generally at 10, a combustionchamber in accordance with the invention, the combustion chamber 10having an enclosed hollow body 11, which is generally circular incross-section. The hollow body 11 has a cylindrical wall section 12, anda top section 13 at end 14 of the cylindrical wall section 12. Thecylindrical wall section 12 narrows at end 15 to form a frusto-conicalsection 16, which terminates in a neck section 17, within which the ashcollects in use to be removed through an exit pipe 18, which isregulated by a valve 19.

A primary air inlet 20 is connected to the neck section 17 at point 21and is regulated by a valve 22 housed therein.

A fuel inlet 23 is mounted in the top section 13 at point 24 and in thisembodiment the fuel inlet 23 also acts as a secondary air inlet.

Four volatiles outlets 25 (one visible) are mounted in the cylindricalwall section 12, adjacent the top section 13. Each volatiles outlet 25has a plurality of apertures 26 of differing sizes arranged in a plate27. The plate 27 is made of tungsten to withstand the heat generated inuse.

A grate 28 is mounted within the hollow body 11 and supports the woodpellets 29 to be burnt. As the wood pellets 29 burn, they break up, fallthrough the grate 28 and are held on a mesh 30 while they burn for afurther period, until they finally fall through the mesh 30 as ash (notshown), to be collected in the neck section 17.

In the embodiment illustrated the combustion chamber 10 forms part of afire chamber, shown generally at 31, in accordance with the invention.The fire chamber 31 has a water jacket 32 having a heat transfer surface33 which encircles the combustion chamber 10. Around the area ofcylindrical section 12, in which the volatiles outlets 25 are mounted,the heat transfer surface 33 is formed so as to create a channel 34between the heat transfer surface 33 and the cylindrical section 12.

The water jacket 32 has a water inlet 35 and a water outlet 36.

In use the wood pellets 29 are introduced into the combustion chamber 10through the fuel inlet 23 at a rate appropriate for the required heatoutput of the device. Primary air at the appropriate pressure isintroduced into the combustion chamber 10 via the primary air inlet 20and is blown up through the mesh 30, the grate 28 and the pellets 29.Thus, the primary combustion takes place above the grate 28 in the areaof the top section 13. Again, depending on the output required,secondary air is introduced into the combustion chamber 10 through thefuel inlet 23 and mixes with the volatiles above the pellets 29. Theburning volatiles then exit the combustion chamber 10 through thevolatiles outlets 25 in a turbulent flow and circulate around thecylindrical section 12 raising the temperature of both the heat transfersurface 33 and the combustion chamber 10 itself.

Thus, the burning of the volatiles is concentrated in the area above thepellets 29 and in the channel 34. The burning volatiles will remain inthis area due to thermal buoyancy until they start to cool. As thevolatiles cool they drop down in the combustion chamber 10 and theexhaust gases are vented through a flue 37, which is regulated by apaddle valve 38.

Referring to FIG. 2 the arrangement of the volatiles outlets 25 aroundthe cylindrical section 12 can be seen more clearly. The arrows 39indicates the path of the turbulent flow of the volatiles through thechannel 34 around the combustion chamber 10. The volatiles outlets 25are offset in the cylindrical section 12 such that the turbulent flow ofvolatiles, as it exits the volatiles outlets 25 is already directedaround the combustion chamber 10, as desired.

Referring to FIG. 3 there is illustrated, generally at 40, a secondembodiment of a combustion chamber in accordance with the invention. Thecombustion chamber 40 is designed to burn wood pellets and to provide ablown flame and is suitable for use as a replacement for an oil burnerin an oil-fired heating boiler.

The combustion chamber 40 has an enclosed hollow body 41, which has agenerally circular cross-section and a domed top section 42. Afrusto-conical section 43 extends laterally from the top section 42 andterminates in a volatiles outlet 44 having a plurality of apertures 45.A fuel inlet 46 is located at point 47 on the top section 42 and aprimary air inlet 48 is located at point 49 on the body section 41.

A secondary air inlet 50 is mounted in the frusto-conical section 43 andis positioned such that a secondary air nozzle 51 is located, within thehollow body 41, adjacent the volatiles outlet 44.

A grate 52 is mounted within the hollow body 41 and supports the woodpellets 53 to be burnt. A drop-on umbrella shaped plate 54 is mountedcentrally on the grate 52 at position 55 below the fuel inlet 46. Inuse, the drop-on plate 54 prevents the burning wood pellets 53 frombeing crushed by fresh pellets 53 dropping from the fuel inlet 46 andalso helps to disperse the pellets 53 over the grate 52.

In use, the burning of the pellets 53 on the grate 52 results in burningvolatiles above the grate 52. These volatiles mixed with the primary andsecondary air are forced through the apertures 45 of the volatilesoutlet 44 and exit as a rapidly burning turbulent flow, which can bedirected onto a heat transferring surface within the fire chamber of theheating boiler.

Referring to FIG. 4 there illustrated generally at 60, a thirdembodiment of a combustion chamber in accordance with the invention, thecombustion chamber 60 having an enclosed hollow body 61, which isgenerally circular in cross-section. The hollow body 61 has acylindrical wall section 62 and a top section 63 at end 64 of thecylindrical section 62.

A volatiles outlet 65 is mounted in the top section 63 and has aplurality of apertures 66 therein. A pipe 67 passes through a centralopening 68 in the top section 63. The pipe 67 serves as a fuel inlet 69and a secondary air inlet 70.

A grate 71 is mounted within the hollow body 61 and supports the woodpellets 72 to be burnt.

A primary air inlet 73 is mounted at the bottom end 74 of the hollowbody 61.

In the embodiment illustrated the combustion chamber 60 forms part of afire chamber, shown generally at 75, in accordance with the invention.The fire chamber 75 has a water jacket 76 having an inner heattransferring surface 77 which encircles the combustion chamber 60. Theinner heat transferring surface 77 has an upper section 78, which has aninverted frusto-conical shape and a lower cylindrical section 79.

A set of upstanding curved formations 80 is mounted on an internalsurface 81 of a top section 82 of the inner heat transferring surface77. The set of upstanding curved formations 80 is arranged equidistantlyaround the combustion chamber 60 and this can be more clearly seen withreference to FIG. 5.

A means (not shown) for moving the combustion chamber along its verticalaxis is provided. Thus, when a low output is required the combustionchamber 60 can be moved closer to the top section 82 of the inner heattransferring surface 77. This has the effect of restricting the flow ofvolatiles through the volatiles outlet 65. Conversely, when a higheroutput is required the combustion chamber 60 can be moved further awayfrom the top section 82 of the inner heat transferring surface 77.

In use the wood pellets 71 are introduced into the combustion chamber 60through the fuel inlet 69 at a rate appropriate for the required heatoutput of the device. Primary air at the appropriate pressure isintroduced into the combustion chamber 60 via the primary air inlet 73and is blown up through the grate 71 and the pellets 72. Thus, theprimary combustion takes place above the grate 71. Again, depending onthe output required, secondary air is introduced into the combustionchamber 60 through the secondary air inlet 70 and mixes with thevolatiles above the pellets 72. The burning volatiles then exit thecombustion chamber 60 through the volatiles outlet 65 in a turbulentflow and circulate around the fire chamber 75 raising the temperature ofboth the inner heat transferring surface 77 and the combustion chamber60 itself.

The shape and positioning of the set of upstanding curved formations 80within the fire chamber 75 causes the volatiles to circulate around thecombustion chamber 60 and also enhances the turbulent flow of thevolatiles. As the volatiles cool they drop down to the end 74 of thecombustion chamber 60, where they pass through an exit pipe (not shown).

What is claimed is:
 1. A combustion chamber for burning solid fuelshaving a high volatiles content, the combustion chamber comprising: anenclosed hollow body in which fuel is to be burned, the body having afuel inlet, an air inlet configured to introduce primary and secondaryair of combustion to the combustion chamber, and a burning volatilesoutlet mounted therein, the burning volatiles outlet having a pluralityof apertures, wherein the burning volatiles outlet is configured torapidly turbulate a confluence of burning volatiles and secondary air ofcombustion as the confluence exits the combustion chamber in a rapidlyturbulating flow resulting in efficient combustion of the volatiles. 2.A combustion chamber according to claim 1, wherein the apertures are ofdiffering sizes.
 3. A combustion chamber according to claim 1, whereinthe burning volatiles outlet includes means for temporarily restrictingthe flow of burning volatiles therethrough at reduced boiler output. 4.A combustion chamber according to claim 1, wherein the air inletprovides primary air of combustion, and a separate inlet adjacent theburning volatiles provides secondary air of combustion within thecombustion chamber.